The present invention relates to integrated gate bipolar transistor (IGBT) modules and more specifically to a method and apparatus for predicting the junction temperatures of IGBTs in an IGBT module operating at a low frequency or a DC condition.
Because of their advantageous operating characteristics (e.g., high switching speeds) IGBTs are used in many different types of power conditioning modules including AC to DC converters, DC to AC inverters, AC-DC-AC converters, etc. For example, in the case of a DC-AC inverter, six IGBTs are arranged to form an inverter bridge along with six diodes.
During switching operations IGBTs and diodes generate heat that has a magnitude related to the switching frequency as well as the amount of current passing through the devices. When IGBT or diode junction temperature exceeds a specific threshold temperature associated with a device type, the devices have been known to fail. In order to reduce failure rate, IGBTs and diodes used in power conditioning modules are typically mounted to heat dissipating devices such as air or liquid cooled heat sinks and are rated for specific current levels/switching frequencies.
It is not possible to measure the temperature of a diode or IGBT junction directly and therefore device junction temperature has to be estimated or predicted. To predict device junction temperature during switching operations, some industry members have identified the thermal impedance associated with each device type and have mounted a temperature sensor (e.g., a negative temperature coefficient sensor (NTC sensor)) to the device (e.g., to a device case as opposed to at the junction itself). Then, during device switching, the measured device temperature and thermal impedance are used to calculate the power losses of the device and hence to predict the IGBT junction temperature. Hereinafter, the method described above to predict junction temperature will be referred to as a conventional prediction method. This method works well in cases where switching devices (e.g., IGBTs, diodes) are thermally isolated from other switching devices (i.e., where devices are mounted on separate heat sinks or are separated by a substantial distance (e.g., three device width dimensions) from other devices on the same sink).
To reduce the space required by the switching devices and diodes as well as the number of heat sinking components, in many cases a single heat sink having a single mounting surface is provided where all of the IGBTs and diodes that comprise a conditioning circuit are mounted to the single mounting surface. Unfortunately, when devices are mounted in close formations on a single heat sink, the conventional prediction method described above has been shown to be inaccurate. In the case of tightly packed devices on a single sink, because one device is extremely close to other devices on the sink, heat form one device tends to heat up adjacent devices. While heat from one device tends to increase the temperature of adjacent devices under all operating conditions, the neighbor heating effect is exacerbated at low switching frequencies and when a conditioning circuit is operated under DC conditions. For instance, in at least some experiments it has been observed that under DC conditions in a six-pack IGBT inverter module, a maximum prediction error of nearly 30 degrees Celsius has occurred when using the conventional prediction method.
In order to avoid device failure due to the prediction error, one solution has been to rate conversion modules (e.g., inverter, converters, etc.) at lower current and switching frequency levels (i.e., are de-rated) than the separate switches used to configure the modules. While this solution substantially eliminates the failure problem, this solution is relatively expensive as circuits including larger and more costly switching devices are required for specific current levels and switching frequencies. In addition, because the switching devices are physically larger, the sinks for mounting the devices are larger and the overall space required to accommodate the conversion modules is increased.